We are interested in studying the mechanism of pH-independent virus-cell membrane fusion that is triggered upon binding of viral envelope proteins to specific receptor(s) on the host's cell surface. Most retroviruses, including HIV and avian leukosis and sarcoma viruses (ALSV), use this mechanism to enter the target cell. The system we are studying is formed by Env-A, the envelope glycoprotein presented on the surface of subgroup A avian leukosis and sarcoma viruses (ALSV-A), and its cellular receptor Tva, expressed on the surface of susceptible avian cells. Tva is the only receptor required to mediate infection by ALSV-A. Our initial goal is to use nuclear magnetic resonance techniques to elucidate the structure of a 47 amino acid fragment of Tva extracellular domain (s47). s47 is both necessary and sufficient to bind Env-A and, when anchored to the cellular membrane, mediate virus-cell membrane fusion. So far, we have been able to completely assign the chemical shift of backbone atoms by using a 15N,13C double labelled sample of s47. Understanding the viral fusion process occurring at the cell surface, independently of pH, may help in developing new therapeutical drugs against viruses, like HIV, that use this mechanism for infecting hosts cells.